Control and ability to rapidly transition from one polyolefin product to another is essential when producing a portfolio of products on a gas-phase polyolefin reactor. During the transition, the flow rate of one or more reactant components (e.g., hydrogen, monomer, co-monomer, co-catalyst and/or catalyst modifier) must be changed. For example, to control and decrease the molecular weight of a polyolefin product, the ratio of hydrogen to monomer must be increased. In other words, the hydrogen concentration within the polyolefin reactor determines the molecular weight of the polyolefin product. Molecular weight is a key, measurable property, which determines physical properties in the polyolefin product. By increasing the hydrogen concentration within the polyolefin reactor, the molecular weight of the produced polyolefin is decreased. Hydrogen serves to terminate the polymerization reaction on the catalyst, and to halt the formation of the polyolefin chain. An increased hydrogen concentration (i.e., decreased monomer concentration) results in shorter polyolefin chains, and in a lower molecular weight polyolefin product.
Conversely, to control and increase the molecular weight of the polyolefin product, the ratio of hydrogen to monomer must be decreased. By increasing the monomer concentration within the polyolefin reactor, the molecular weight of the produced polyolefin is increased. An increased monomer concentration (i.e., decreased hydrogen concentration) results in longer polyolefin chains, and in a higher molecular weight polyolefin product.
To change other physical properties of the polyolefin product, the co-monomer, co-catalyst and/or catalyst modifier concentration must be changed. For example, in impact co-polymer (ICP) polypropylene production, the ethylene concentration within the polyolefin reactor must be changed to achieve the specific properties for various ICP grades. A change in ethylene concentration results in an altered rubber content in the produced polyolefin product.
In other words, each polyolefin product has specific physical properties that define that grade. If a produced polyolefin does not have these physical properties, it is an off-grade material with an inherently low value. During the transition, the produced polyolefin does not have the physical properties of either the starting grade or the ending grade. The polyolefin produced during the transition is a low-value, off-grade material.
Thus, a system and method is needed to minimize the transition period and to reduce the production of off-grade material during transitions.